The cross linkage, for example with sulfur, of polymers including insaturations, notably co-polymers of styrene and conjugated diene such as butadiene within the bitumen, lead to bitumen/polymer compositions having improved stability and excellent physico-mechanical properties. These bitumen/polymer compositions can be used for producing surfaces that have improved characteristics over those obtained from bitumens alone. The preparation of bitumen/polymer compositions of the above type for which the polymer able to be cross-linked using for example sulfur, is a copolymer of styrene and a conjugated diene is notably disclosed in FR2376188, FR2528439, EP0360656 and EP0299820.
The bitumen/polymer compositions from which, after dilution, bitumen/polymer binders are produced are also called “bitumen/polymer concentrates”. These bitumen/polymer concentrates are more economical to prepare and transport than diluted bitumen/polymer compositions having a lower polymer content (as a lesser amount to transport means lower transport costs). Such concentrate can be diluted at the time of use, by adding bitumen, to obtain bitumen/polymer binders having a lower polymer content generally employed for producing surfacings. The bitumen/polymer concentrates are prepared with a polymer content below a certain threshold which can vary from about 3 to 6% by weight of bitumen. Indeed, once the polymer content of the bitumen/polymer concentrates exceeds the above threshold, there is a significant risk of the bitumen/polymer concentrates gelling in the course of their preparation or their storage. It is consequently difficult, in practice, to produce non gelling bitumen/polymer concentrates having a high polymer content. Furthermore, the bitumen/polymer concentrates are more viscous and consequently difficult to pump.